How to tell if an object is linked with another blend?
I was looking for all sorts of variable names in the reference, 'link', 'blend', etc. I figured out that object.library was what I was looking for. It will return None if it's not from a library, otherwise it will return a bpy.types.Library

https://www.blender.org/api/blender_python_api_2_60_6/bpy.types.Library.html#bpy.types.Library